CPAP treatment is a common ameliorative treatment for breathing disorders including OSA. CPAP treatment, as described in U.S. Pat. No. 4,944,310, provides pressurised air or other breathable gas to the entrance of a patient's airways at a pressure elevated above atmospheric pressure, typically in the range 4-20 cm H2O.
It is also known for the level of treatment pressure to vary during a period of treatment in accordance with patient need, that form of CPAP being known as automatically adjusting nasal CPAP treatment, as described in U.S. Pat. No. 5,245,995.
NIPPV is another form of treatment for breathing disorders which can involve a relatively higher pressure of gas being provided in the patient mask during the inspiratory phase of respiration and a relatively lower pressure or atmospheric pressure being provided in the patient mask during the expiratory phase of respiration.
In other NIPPV modes the pressure can be made to vary in a complex manner throughout the respiratory cycle. For example, the pressure at the mask during inspiration or expiration can be varied through the period of treatment, as disclosed in the applicant's international PCT patent application No. PCT/AU97/00631.
Typically, the ventilatory assistance for CPAP or NIPPV treatment is delivered to the patient by way of a nasal mask. Alternatively, a mouth mask or full face mask or nasal prongs can be used. In this specification any reference to a mask is to be understood as incorporating a reference to a nasal mask, mouth mask, full face mask or nasal prongs.
In this specification any reference to CPAP treatment is to be understood as embracing all of the above described forms of ventilatory treatment or assistance.
Breathable gas supply apparatus broadly comprise a flow generator constituted by a continuous source of air or other breathable gas generally in the form of a blower or driven by an electric motor. The electric motor driving the blower is typically controlled by a servo-controller under the control of a micro controller unit. A hospital piped supply can also be used. The gas supply is connected to a conduit or tube, which in turn is connected to a patient mask which incorporates, or has in close proximity, an exhaust to atmosphere for venting exhaled gases.
In order to prevent drying of the patient's airways during use of a breathable gas supply apparatus it is known to pass the gas through a humidifier before supplying the gas to the patient. Humidification is achieved by passing the air over a water surface within a humidifier so that the gas absorbs moisture from the water before being delivered to the patient. The two main types of humidifier are passive, where the water is not heated, and active, where the water is heated.
A known passive humidifier is the applicant's Passover (Trade Mark) humidifier. Other passive humidifiers are disclosed in U.S. Pat. Nos. 5,231,979, 5,537,997 and 5,598,837.
A known active humidifier is the applicant's HumidAire (Trade Mark) which heats the water in the humidifier via a thermostatically controlled electric heating element. The temperature of the water is set manually by the patient. The HumidAire humidifier also has a built in safety feature to prevent the heater element reaching excessive temperatures under fault conditions.
European patent application No. EP 0845277 discloses an active humidifier that includes a thermostatically controlled electric heating plate. Active humidifiers are also disclosed in U.S. Pat. Nos. 4,621,632 and 4,203,027.
A disadvantage of known active humidifiers is the expense of the heating elements and associated thermostatic temperature control equipment.
A disadvantage of known passive humidifiers is an inability to easily vary the amount of humidification of the gas.
It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages.